Persons who work in polluted environments commonly wear filtering face masks to protect themselves from inhaling airborne contaminants. Filtering face masks typically have a fibrous or sorbent filter that is capable of removing particulate and/or gaseous contaminants from the air. When wearing a face mask in a contaminated environment, wearers are generally comforted with the knowledge that their health is being protected, but they are, however, contemporaneously discomforted by the warm, moist, exhaled air that accumulates around their face. The greater this facial discomfort is, the greater the chances are that wearers may temporarily remove the mask from their face to alleviate the unpleasant condition. To reduce the occurrence of such an event, manufacturers of filtering face masks often install an exhalation valve on the mask body to allow the warm, moist, exhaled air to be rapidly purged from the mask interior. The rapid removal of the exhaled air makes the mask interior cooler and hence makes the mask more comfortable to wear, which in turn benefits worker safety because mask wearers are less likely to remove the mask from their face to eliminate the hot, moist, environment that accumulates around their nose and mouth.
For many years, commercial respiratory masks have used “button-style” exhalation valves to purge exhaled air from mask interiors. The button-style valves have employed a thin circular flexible flap as the dynamic mechanical element that lets exhaled air escape from the mask interior. The flap is centrally mounted to a valve seat through a central post. Examples of button-style valves are shown, for example, in U.S. Pat. Nos. 2,072,516, 2,230,770, 2,895,472, and 4,630,604. When a person exhales, a circumferential portion of the flap is lifted from the seal surface of the valve seat to allow air to escape from the mask interior.
Button-style valves have represented an advance in the attempt to improve wearer comfort, but investigators have made other improvements, an example of which is shown in U.S. Pat. No. 4,934,362 to Braun. The valve described in this patent uses a parabolic valve seat and an elongated flexible flap. Like the button-style valve, the Braun valve also has a centrally-mounted flap and has a flap edge portion that lifts from a seal surface during an exhalation to allow the exhaled air to escape from the mask interior.
After the Braun development, another innovation was made in the exhalation valve art by Japuntich et al.—see U.S. Pat. Nos. 5,325,892 and 5,509,436. The Japuntich et al. valve uses a single flexible flap that is mounted off-center in cantilevered fashion to minimize the exhalation pressure that is required to open the valve. When the valve-opening pressure is minimized, less power is required to operate the valve, which means that the wearer does not need to work as hard to expel exhaled air from the mask interior when breathing. Other valves that have been introduced after the Japuntich et al. valve also have used a non-centrally mounted cantilevered flexible flap—see U.S. Pat. Nos. 5,687,767, 6,047,698, and RE 37,974E. Valves that have this kind of construction are sometimes referred to as “flapper-style” or “cantilevered” exhalation valves.
In yet another development, the exhalation valve has been provided with a flexible flap that includes first and second juxtaposed layers, where at least one of the layers is stiffer, or has a greater modulus of elasticity, than the other layer—see U.S. patent application Ser. No. 09/989,965 to Martin et al. In a preferred embodiment of this exhalation valve, the flap layer that is the softer, more flexible (less stiff) layer, which has the lower modulus of elasticity, is disposed on the portion of the flexible flap that makes contact with the valve seat's seal surface. The use of a more flexible layer in this location on the flap allows a better seal to occur between the flexible flap and the seal surface under neutral conditions, that is, when a wearer is neither inhaling nor exhaling. The use of the multilayered flap also can allow a thinner and more dynamic flexible flap to be used in some instances, which then can allow the valve to open easier under less pressure drop to enable warm, moist, exhaled air to escape from the mask interior under less exhalation pressure. Wearers therefore may be able to purge larger amounts of exhaled air from the interior gas space more rapidly without expending as much power, resulting in improved comfort to the mask wearer.
In known valve products, like the exhalation valves described above, the valve seat has consistently been described as having a seal surface material that is relatively rigid in construction. For example, in U.S. Pat. No. 4,934,362 to Braun and in U.S. Pat. No. 5,509,436 to Japuntich et al., the whole valve seat is described as being made from an injection molded plastic. Commercially available products, like the Ventex™ valve sold by Moldex-Metric Inc. have similarly used a rigid plastic to fashion the totality of the valve seat. Although known exhalation valve products have been successful at improving wearer comfort by encouraging exhaled air to leave the mask interior more easily, none of the known valve products have used a resilient seal surface on the valve seat, which, as described below, may provide further benefits towards improving valve performance and hence wearer comfort.